Vegetation-Climate Interactions Along A Transition From Tundra To Boreal Forest In Alaska

Abstract

The climate of the Alaskan Arctic is warming more rapidly than at any time in the last 400 years. Climate changes of the magnitude occurring in high latitudes have the potential to alter both the structure and function of arctic ecosystems. Structural responses reflect changes in community composition, which may also influence ecosystem function. Functional responses change the biogeochemical cycling of carbon and nutrients. We examined the structural and functional interactions between vegetation and climate across a gradient of vegetation types from arctic tundra to boreal forest. Canopy complexity combines vegetation structural properties such as biomass, cover, height, leaf area index (LAI) and stem area index (SAI). Canopy complexity determines the amount of the energy that will be available in an ecosystem and will also greatly influence the partitioning of that energy into different land surface processes such as heating the air, evaporating water and warming the ground. Across a gradient of sites in Western Alaska, we found that increasing canopy complexity was linked to increased sensible heating. Thus, vegetation structural changes could represent an important positive feedback to warming. Structural changes in ecosystems are linked to changes in ecosystem function. High latitude ecosystems play an important role in the earth's climate system because they contain nearly 40% of the world's reactive soil carbon. We examined Net Ecosystem Production (NEP) in major community types of Northern Alaska using a combination of field-based measurements and modeling. Modeled NEP decreased in both warmer and drier and warmer and wetter conditions. However, in colder and wetter conditions, NEP increased. The net effect for the region was a slight gain in ecosystem carbon; however, our research highlights the importance of climate variability in the carbon balance of the study region during the last two decades. The next step forward with this research will be to incorporate these results into coupled models of the land-atmosphere system. Improved representations of ecosystem structure and function will improve our ability to predict future responses of vegetation composition, carbon storage, and climate and will allow us to better examine the interactions between vegetation and the atmosphere in the context of a changing climate.